The summer's temperature generally ranges from 10 to 15 Celsius over night, and 16 to 23 degree in the day time. Occasionally, it may rise to 40 for a couple of hours in the late afternoon, but it will quickly cool down after sunset. 

During our stay in Geelong, one of the night temperature even dropped down to 9 degree on 14 Dec 97. It was freezing during summer.  Always bring along a jumper with you as the weather can change swiftly especially in the coastal areas.